Trying to use Sprite Renderer to swap between 2 sprites

I'm trying to get a sprite to toggle between its on and off sprite (a ball that shoots a laser and a ball that doesn't shoot a laser) at random intervals.

The problem is I don't know how to get past sprite renderer. The problem is in my fixedupdate code.

 var interval: float = 0.5f;  //random time to turn off and on each laser generated   
 var rotationSpeed: float = 0.0f; //makes lasers rotate
 var isLaserOn: boolean = true;  //tests if our laser is on  
 var timeUntilNextToggle: float; //time to toggle to the next on/off state
 
 function Start () 
 {
     timeUntilNextToggle = interval; //will set the time until the laser will toggle its state for the first time
 }
 
 //function Update () 
 
 
 function FixedUpdate () 
 {
     //Decreases time until next toggle
     timeUntilNextToggle -= Time.fixedDeltaTime;
  
     //If time is 0 or less then the laser will toggle states 
     if (timeUntilNextToggle <= 0) 
     {
  
         //sets the correct state of the laser
         isLaserOn = !isLaserOn;
  
         //if our laser is on the collider is activated, if it is off then there is no collider to interfere with the player
         collider2D.enabled = isLaserOn;
  
         //will switch between our two sprite images
        // SpriteRenderer.sprite = ((SpriteRenderer)this.renderer);
        var newSprite : Sprite;
          GetComponent(SpriteRenderer).sprite = newSprite;
      
         if (isLaserOn)
             spriteRenderer.sprite = laserOnSprite;
         else
             spriteRenderer.sprite = laserOffSprite;
  
         //resets time until next toggle variable since it has just been toggled
         timeUntilNextToggle = interval;
     }
  
     //rotates our lasers around the z axis (set rotationspeed to 0 to disable in the inspector)
     transform.RotateAround(transform.position, Vector3.forward, rotationSpeed * Time. fixedDeltaTime);
 }